graphic_03.eps East winds - positive feedback to other sources of warming.

positive feedback to other sources of warming.

2.02 graphic_03.eps East winds

East winds Trade winds Trade winds Meridional circulation East winds West winds H H H L L L L H H West winds

Figure 9. Generalized representation of circulation patterns (WMO 2003, p. 46).

Figure 10 demonstrates that the effects of El Niño episodes (warm events) and the cool events, known as La Niña, occur across the entire globe.

There are also longer-term changes in ocean-atmosphere circulation − marked by shifts in the location and/or intensity of the semi-permanent high- and low-pressure cells. These changes can persist for several decades. For example, temperature and circulation patterns in the North Pacifi c appear to get “stuck” in one of two modes for long periods. Various indices provide measures of this tendency, but they all strongly depend on the intensity and position of the winter Aleutian low-pressure system. Figure 11 displays one such index: the Pacifi c Decadal Oscillation (PDO) Index. When the PDO is in its positive coastal warm phase, as it was for most of the period from 1977 through the mid-1990s, sea surface temperatures along the west coast of North America are unusually warm, the winter Aleutian low intensifi es, and the Gulf of Alaska is unusually stormy.

Figure 10. Expected seasonal effects of El Niño (warm episodes) across the globe during December− February (top) and expected seasonal effects of La Niña (cold episodes) during the same time period (bottom) (from Climate Diagnostics Center, NOAA).

The slowly evolving state of the ocean, as measured by the PDO, interacts with the more rapid ENSO-related changes to inﬂ uence storm tracks and, thus, the likelihood of unusually heavy or light seasonal precipitation. For example, a positive PDO appears to reinforce the effects of an El Niño, making wet winter conditions in the southwestern United States and dry conditions in the Paciﬁ c Northwest more likely than would be the case if the PDO were in the negative (coastal cool) phase.

A similar pattern of multi-year variability occurs in the Atlantic basin as well. The North Atlantic Oscillation (NAO) measures swings in the relative intensity of the winter low-pressure cell centered over Iceland, and the high-pressure cell centered over the Azores. The NAO is in a positive phase when that pressure difference is larger than normal. A positive NAO pattern drives strong, westerly winds over northern Europe, bringing warm stormy winter weather, while southern Europe, the Mediterranean and Western Asia experience unusually cool and dry conditions (Figure 12a). Also in the positive phase, northeastern Canada is more likely to experience unusually cold winter conditions. In the negative phase, the pressure differential is smaller than average and winter conditions are unusually cold over northern Europe and milder than normal over Figure 11. Paciﬁ c Decadal Oscillation. Upper panel: sea surface temperature and wind stress anomalies. Lower panel: Monthly values of PDO Index. Red is coastal warm phase; blue is coastal cool phase (courtesy of Dr. Nathan Mantua, JISAO, University of Washington and Stephen Hare, International Paciﬁ c Halibut Commission).

Greenland, northeastern Canada, and the Northwest Atlantic. There have been long periods during which the NAO has tended to be either unusually low or unusually high. In particular, it was generally low throughout the 1950s and 1960s, and then abruptly switched to a positive state for most of the period from 1970 to the present (Figure 12b).

ENSO, the PDO, and the NAO are all natural modes of climate variability, but any change in global climate is also likely to affect these processes. At the global scale,

climate changes depend on changes in the Earth’s energy budget. In particular, increased concentrations of greenhouse gases, such as carbon dioxide, in the atmosphere are likely to cause warmer global average surface temperatures.

Figure 12a. Schematic of the positive index phase of the North Atlantic Oscillation (NAO) during the Northern Hemisphere winter (courtesy of Dr. James Hurrell, CGD/NCAR).

The Earth’s climate has changed throughout geologic time − why did those changes occur?

There is strong evidence that the Earth has experienced long periods during which average global temperatures were much colder and much warmer than today. Changes in the Earth’s climate system throughout geologic time can be linked to changes in the components of the climate system, including changes in the Earth itself, the

composition of the atmosphere, and the seasonal distribution and total amount of incoming solar energy.

There have been enormous changes in the surface of the Earth − with continents moving, mountain ranges growing and eroding away, and the area covered by oceans and by ice growing or shrinking. The composition of the atmosphere has also changed as a result of biological and geophysical processes, including storage of carbon in the ocean and its subsequent release, volcanic eruptions,

and the occasional sudden release of methane from sediments on the ocean ﬂ oor. In addition, there have been changes in solar output, in the Earth’s orbit, and Earth-Sun geometry. All of these changes affect climate at both the global and regional scale.

Consider, for example, the effects of slow changes in the Earth’s orbit around the Sun. Over the course of approximately 100,000 years, the Earth’s orbit around the Sun changes shape from a thin oval to a circle, and back again. At present, the shape of the Earth’s orbit is almost a perfect circle. There is only a small difference in our distance from the Sun at the time when we are closest to it (the perihelion, currently in January), and when are farthest away (the aphelion, currently in July). The fact that the Earth is now closest to the Sun during the northern hemisphere winter is just a coincidence, because the date of the perihelion slowly moves to come later in the year, following a 21,000-year cycle. In other words, 10,000 years from now, the perihelion will occur in the northern hemisphere summer, causing northern hemisphere seasonal contrasts to be somewhat more pronounced than at present (Figure 13).

Figure 13. Graphic illustration of the Earth’s orbit and average solar radiation comparing present conditions with those 9 thousand years ago (9ka)(Trenberth et al. 1999).

Even such subtle differences can have profound impacts on regional climates. When the perihelion last occurred in the northern hemisphere summer, the Sahara was much wetter than it is now and was covered with savanna-like vegetation. As the seasonal distribution of solar radiation gradually changed to modern conditions, the Sahara dried out. Its transformation to the present-day desert accelerated dramatically about 5,500 years ago. The abruptness of the change suggests that the climate system crossed a threshold, triggering a series of biophysical feedbacks that ampliﬁ ed the trend toward regional drying (IGBP 2001).

Seasonal contrasts would also tend to be more extreme when the shape of the Earth’s orbit is more elliptical than it is at present. In addition, the Earth wobbles slightly on its axis, so that the angle of the tilt varies over a 41,000-year cycle. Recall that the Earth’s tilt causes seasons in the ﬁ rst place. So, the greater the angle of tilt, the stronger the seasonal contrasts. These astronomical Milankovich cycles appear to have played a signiﬁ cant role in the timing of ice ages and interglacial periods in the recent past, but they clearly cannot explain all of the Earth’s climate history.

Changes in the seasonal distribution of incoming solar energy may have triggered the beginning and end of previous ice ages. However, the solar impacts were greatly ampliﬁ ed by positive feedbacks within the climate system, including changes in the reﬂ ection of sunlight back into space by ice-covered areas, changes in ocean circulation, and dramatic changes in atmospheric concentrations of greenhouse gases, especially carbon dioxide and methane. Over the past 400,000 years, the record of temperatures in the world’s high-latitude regions followed a saw-toothed pattern. Global concentrations of carbon dioxide and methane followed

a nearly identical pattern (Figure 14). There were four long but erratic periods of cooling, each followed by a dramatic warm- up. Scientists do not fully understand the reasons for this pattern, but changes in the ocean’s thermohaline circulation (Figure 15) and changes in the release of carbon dioxide from the oceans, and the release of methane from wetlands, appear to have

Figure 14. Four glacial cycles are recorded in Vostok ice cores. The graphic represents thousands of years before the present. The top three lines from the Vostok ice core record show Deuterium – a proxy for local temperature (blue); CO₂ (black); methane (red); and dust (purple). The green line is a measure of Chinese loess deposition. (after ﬁ gure compiled by the PAGES program; K. Alverson et al., 2003)

played important roles. In Figure 14, one can see that rapid warming and increases in atmospheric carbon dioxide and methane occurred nearly simultaneously. This suggests a positive feedback loop, with initial warming causing the greenhouse gas concentrations to rise, and rising concentrations promoting further warming. Figure 14 also shows a correspondence between the temperature record and long periods of wet or dry conditions in Central and East Asia. Wind-borne dust deposits, both in Antarctica (Vostok) and on the Chinese Loess Plateau tended to peak during glacial periods, indicating expansion of Asian deserts.

Figure 15 depicts the approximate pattern of thermohaline circulation in the World’s oceans − that is, the connection between the movement of cold, salty water in the oceans’ depths and the movement of warm, less saline water at the surface (Broecker 1997). Warm, low-salinity water from the tropical Pacifi c and Indian Oceans fl ows around the tip of South Africa and ultimately joins the Gulf Stream to transport heat from the Caribbean to Western Europe. As the water moves northward, evaporative heat loss cools the water and leaves it saltier and more dense. The cold, salty water sinks in the North Atlantic and fl ows back toward Antarctica, thus pushing the conveyor along. One hypothesis is that the infl ow of fresh water into the North Atlantic during warm periods can cause this conveyor to dramatically slow down or even collapse. Such a mechanism could explain the sudden reversals of warming that appear in the geologic record.

It is likely that increased high-latitude runoff and ice-melt caused by human-induced climate change will slow the thermohaline circulation. However, we do not know how much that would reduce projected temperature increases for Europe and the northern latitudes, because the mechanisms of human-induced climate change are different from the mechanisms of previous natural warming episodes (IPCC WG I 2001). This is an area of active research.

Why should I believe that emissions of carbon dioxide and other greenhouse gases will cause global climate change?

The major greenhouse gases, carbon dioxide, methane, nitrous oxide and water vapor, occur naturally in the atmosphere. Without them, the Earth would be too cold to support life as we know it. The basic science of the greenhouse effect is not controversial. Scientists understand the greenhouse effect and can easily reproduce it in the laboratory. There is no disagreement about the fact that these gases are transparent to incoming short-wave solar radiation, and that they tend to absorb outgoing long-wave radiation and re-emit part of that radiation back down to the Earth’s surface. In effect, they act as a blanket to warm the surface of the Earth.

Concern about climate change arises from the fact that human activities are releasing large quantities of these substances − and other even more powerful manufactured greenhouse gases such as halocarbons − into the atmosphere (Table 2). Because carbon

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Chemical Annual Annual spheric 100-yr Chemical Annual Annual spheric 100-yr Species Formula Abundance % Change Emission Lifetime GWP Species Formula Abundance % Change Emission Lifetime GWPc

(units) 2002 1750 1990s late 1990s (yr)

Carbon CO₂ (ppm) 372 280 0.4 % 6.3 +/ ~5 to dioxide 22 - 0.4 PgC 200 1 Methane CH₄ (ppb) 1729d₇₀₀ _{0.4 %} _{600 Tg} ₁₂a ₂₃ Nitrous oxide N₂O (ppb) 314 270 0.3 % 16.4 TgN 114a ₂₉₆ Perfl uoromethane CF₄ (ppt) 80 40 1.3 % ~15 Gg >50000 5700 Perfl uoroethane C₂F₆ (ppt) 3.0 0 2.7 % ~2 Gg 10000 11900 Sulphur SF₆ (ppt) 4.2 0 5.7 % ~6 Gg 3200 22200 hexafl uoride HFC-23 CHF₃ (ppt) 14 0 3.9 % ~7 Gg 260 12000 CFC-11b _CFCl 3 (ppt) 268 0 -0.5 % 45 4600 CFC-12b _CF 2Cl2 (ppt) 533 0 0.8 % 100 10600 Sources: IPCC WGI 2001; Blasing and Jones 2003.

a_{Species with chemical feedbacks that affect the duration of atmospheric response – values are perturbation lifetimes} b_{Regulated under Montreal Protocol}

c_{Global Warming Potential (GWP) is an index describing the relative effectiveness of well-mixed greenhouse}

gases in absorbing outgoing infrared radiation. The index approximates the time-integrated warming effect of a unit mass of a given greenhouse gas relative to that of carbon dioxide.

d_{As measured at Cape Grim, Tasmania (Blasing and Jones 2003).}

Trend- Atmo-